The invention relates to a method and system for applying return torque in a vehicle power steering system.
Some vehicle power steering systems use an actuator coupled to the steering shaft to provide power assist in vehicle steering. It is often desirable in such systems to provide a return torque command to the actuator so as to help return the steering gear to a centered position and provide a stable steering feel to the vehicle operator. Such a return torque command is generated as a function of steering position, provided the steering position is known. Rotational position sensors having high resolution and other desirable characteristics may be coupled to the steering shaft to provide an output voltage varying in a substantially linear manner from a first voltage to a second voltage through a rollover angle comprising an entire 360 degree rotation or some sub-multiple thereof. The steering shaft, however, typically rotates through a plurality of such revolutions or sub-multiple angles, and therefore a plurality of repetitions of the rollover angle, as the rack is moved from one end to the other of its operational range. Thus, a sensor directly coupled to the steering shaft for maximum resolution in the output signal is similarly rotated through a plurality of repetitions of the output voltage range and produces a rollover transition at the end of each repetition when the voltage jumps from one extreme to the other. The output of a sensor so coupled does not by itself provide a unique absolute steering position, rather, it provides a set of steering positions separated by a rollover angle corresponding to the full output voltage range. It is thus necessary, in order to know the absolute rotational position, to compensate the sensor output signal for such rollover transitions in order to locate the absolute center position.
Once such compensation utilizes known relationships between hand wheel torque and absolute hand wheel position for given speeds. Based on vehicle dynamics, an envelope relating hand wheel torque and absolute hand wheel position is derived for one or more vehicle speeds. Once a given speed is reached, the hand wheel torque (measured through torque sensors) and the hand wheel position (derived from a position sensor) is compared to the envelope. If the hand wheel torque and hand wheel position are outside the envelope, then a correction is applied until the proper position is found.
The relationships between hand wheel torque and absolute hand wheel position are derived based on predetermined vehicle dynamics and driving conditions. When the driving conditions change from those used to establish the envelope, the hand wheel torque and hand wheel position may not provide accurate results. For example, hand wheel torque when driving on pavement will be different than hand wheel torque when driving on ice. An envelope derived based on driving on pavement may not be accurate under other conditions such as driving on ice.
An exemplary embodiment of the invention is a method for controlling application of return torque in a vehicle power steering system. The method includes detecting a current hand wheel center and comparing the current hand wheel center to a previous hand wheel center. A change in hand wheel center is detected in response to the comparison. Application of return torque is controlled in response to detecting a change in hand wheel center.